This invention relates to an improved methodology for producing phases of carbon nitride (CN, C.sub.2 N.sub.2, C.sub.3 N.sub.4, . . . , C.sub.x N.sub.y) and particularly to methodology for producing films and coatings of .beta.-C.sub.3 N.sub.4.
Because of its previously unmatched physical properties, and particularly its hardness and wear resistance, diamonds and diamond or diamond like coatings have extremely important industrial applications. Diamond's hardness is determined by its bulk modulus which depends on the type of chemical bonding it has, which is covalent rather than ionic, and of the shortness of its bond length. Despite its otherwise impressive wear resistance when used as a coating, diamond has a serious failing in high temperature reactive environments: it is subject to graphitization degradation or oxidation at temperatures above .apprxeq.750 deg. C. Indeed, the maximum operational temperature limit for diamond coated tools is usually lower than 300-400 deg C. Additionally, when used with cobalt compounds diamond is also subject to catalytic degradation. This degradation at elevated temperatures renders diamond coatings unusable in many real world applications, such as a coating for the turbine blades of a jet engine and high speed/high temperature tooling. Recently developed "superhard" materials such as cubic boron nitride (BN) and silicon nitride (Si.sub.3 N.sub.4) are less hard than diamond due to their partially ionic bonds and longer bond lengths. The present application is directed to a process for producing, in a commercially usable manner, a material that has a hardness comparable to that of diamond but is also usable in environments of above 750 deg. C. which would destroy a diamond coating and through 1000 deg C.
The present invention is directed to methodology for producing the various phases of carbon nitride and particularly the so called .beta.-C.sub.3 N.sub.4 phase. Beta carbon nitride or .beta.-C.sub.3 N.sub.4 means carbon nitride that has the same configuration as .beta.-Si.sub.3 N.sub.4 (beta silicon nitride) with the silicon replaced by carbon. As used in this patent application carbon nitride means any of the multiple phases of C.sub.x N.sub.y, .beta.-C.sub.3 N.sub.4 and amorphous carbon nitride unless explicitly stated otherwise.
In a scientific paper based on theoretical calculations (Science, 25 Aug. 1989, p. 841) it was estimated that carbon nitride in the .beta.-C.sub.3 N.sub.4 configuration would have a bulk modulus (hardness) on the order of, or perhaps greater than, diamond. Naturally, this engendered many attempts to grow films of carbon nitride by many different methods. These methods include: pulsed laser ablation (Science, 16 Jul. 1993 p. 334), hot press sintering, shock combustion, reactive sputtering (U.S. Pat. No. 5,111,679), plasma arcs, E-beam evaporation and chemical vapor deposition. However these approaches have met with only limited success as they have not produced films of carbon nitride that have the correct bonding configuration between carbon and nitrogen for .beta.-C.sub.3 N.sub.4 : C-N, single sp.sup.3 bonding. The films produced by these methods have only a small percentage of single sp.sup.3 bonds and/or are subject to contamination by hydrocarbons, most likely caused by hydrogen containing compounds used as reactants. We submit that these processes suffer from a common flaw in that they do not provide proper ion particle energy (which can be measured in particle electron volts) to form films of carbon nitride having a large majority (&gt;90%) of the correct sp.sup.3 bonds. Most of the processes previously used have particle energies of 20-80 eV while formation of the desired metastable carbon nitride state requires particle energies of 100-200 eV.
In U.S. patent application 08/328,806 filed Oct. 25, 1994, now U.S. Pat. No. 5,573,864 and assigned to the National Institute of Standards and Technology there is disclosed a process for producing carbon nitride using a N.sup.+ ion beam and a sputtered C source. However, they were unable to effectively explore the important 100-200 eV energy range and their C source was not energy controlled. In a paper presented to the ASTM/STLE Tribology conference entitled "Synthesis and Tribological Properties of Carbon Nitride as a Novel Superhard Coating and Solid Lubricant" (ASTM/STLE Tribology Transactions Vol. 36(1993),3, pp 491-493) a process for producing carbon nitride using sputtering of a graphite target in an nitrogen/argon plasma is disclosed. However in this process, as in all sputter processes, there is little control over the polarity and energy of the carbon, as there is a shower of C.sup.-, C.sup.+ and C ions and atoms of various energy levels, which prevents precise control of the process of forming the carbon nitride.
The present application is directed to a process for depositing carbon nitride films on substrates or work pieces by means of plasma assisted energy controlled carbon ion beam deposition. This technique has produced microscopically smooth, nearly stress free, insulating, and transparent carbon nitride thin films at room temperature, which contains approximately 95% pure C-N single bonds and approximately 50% atomic % of nitrogen. The friction coefficient and wear resistance of the films produced by the present process are comparable to that of diamond. Furthermore, the films produced do not show any deterioration or decrease in the number of pure sp.sup.3 single C-N bonds after heating to 1000 deg. C in a vacuum or oxygen atmosphere, conditions that decompose a diamond or diamond/cobalt composites.
In the process according to the invention, the substrate or tool to be coated is placed in a vacuum chamber at room or elevated temperature and is acted upon by a source of negative carbon ions and a high flux plasma source containing nitrogen radicals. The source of C.sup.- ions is preferably hydrogen free and capable of providing particle energies in the 100-200 eV range which is required to produce films of true beta carbon nitride. The source of the nitrogen flux should be capable of providing a high density of nitrogen radicals to interact with the C.sup.- ion beam and thereby coat the substrate with carbon nitride. In a further embodiment of the process, which provides an even higher deposition rate, a source of N.sup.+ is added which provides charge neutralization during deposition and can also be used for surface nitridation prior to deposition.